Stephanos, and Zoorzoor stand out because of their antiquity. The St. Thaddeus Church is considered one of the oldest churches in the world, whose construction began 1700 years ago. Historians believe that the Church is the tomb of St. Thaddeus who is said to have been one of Christ's disciples who traveled to Armenia, then part of the Persian Empire, for preaching the teachings of Christ. Armenians followed Thaddeus' teachings and converted to Christianity in 301 AD. St.Thaddeus was later martyred and buried in the present-day West Azarbaijan province. A tomb was erected on his burial place by his followers who turned it into a small prayer house. The building was later changed into a cathedral in the seventh century AD. According to the inscriptions remained there, the Church was ruined in by a devastating earthquake but was later restored in its current form by a Christian religious figure. Today the church belongs to the Armenian community of Iran. It has an international reputation and hosts annual meetings of world Armenians each year in July-August. The present cruciform building, said to be on the site of this early church, stands on a hill within fortified walls and consists of two distinct parts: a domed sanctuary end built largely of dark stone, probably dating from the tenth or eleventh century, and the main body of the church, built of light sandstone, under a second and larger tent dome whose twelve-sided drum is pierced by an equal number of windows.

According to an inscription dating 1329 this latter section was rebuilt after an earthquake in 1319; considerable additions were, however, made during the 19th century, possibly when there was an abortive move to transfer here from Echmiadzin in present Armenia the seat of Armenian Catholics. The exterior walls are, like those of other early Armenian churches, decorated with bas-reliefs, the effigies of saints and a lively frieze of vine leaves and animals on the newer building being particularly striking. Ruined buildings within a walled compound adjoining the western fortified walls indicate that a considerable monastic settlement once existed there. The church has one service a year, on the feast day of St. Thaddeus (around 19 June), when Armenian pilgrims from all over Iran camp for three days to attend the ceremonies.

Because of special features, antiquity, architectural style, decorations, its religious importance among the world Armenians, and the celebrations held annually in Qara Kelisa, Iran's Armenian Monastic Ensembles have been designated as one of the country's sites to be inscribed onto the UNESCO's World Heritage List.